I nominate Freespace 2 in doing so, also nominating FSO, and all of the amazing stuff that has come of it (seriously, look into it, it will blow your mind to see what happens when source code is freed to a community that gives a fuck) Also, mechwarrior 3.
TRIBES 2 for the PC. I consider this game to one of the most underrated, considering it had crazy large map play (way larger than quake or half life), everyone in the game could run around and fly to battle or fight in mid air, and there are ground and air vehicles. The armor and weapon classes were flawless and created a very equal battle environment as well. Enemy Territory is also another great one, though I haven't played it in years. and of course Diablo II